The highest sensitivities ever have been reached and maximum frequencies up into the terahertz range are attained with such diodes. The relatively high cutoff voltage of the diodes in the forward direction, which is typically in the range of 0.7 V to 0.8 V due to electronic surface states in the GaAs, is a disadvantage of these components. To reach sufficient performance capacity for mixer applications in terms of conversion loss, pumping capacity, sensitivity and noise behavior, such diodes are therefore operated with bias voltages (e.g., S. A. Maas: Microwave Mixers, 2nd edition, Artec House, Boston, 1993).
However, operation with a bias voltage leads to disturbances known as Townsend current hum. On the other hand, operation without bias voltage requires a high pumping capacity of the local oscillator (LO) to reach a low conversion loss.
It has been known from theoretical calculations from U. V. Bhapkar, T. A. Brennan and R. J. Mattauch: InGaAs Schottky Barrier Mixer Diodes for Minimum Conversion Loss and Low LO Power Requirements at Terahertz Frequencies, Proceedings of the 2nd international Symposium on Space Terahertz Technology, Feb. 1991, pp. 371-388, that a marked reduction in the necessary LO power for pumping the mixer diode can be expected from Schottky diodes with reduced electron release barriers at InGaAs depletion zone layers compared with GaAs depletion zone layers at comparable conversion losses. In addition, structures consisting of InGaAs layers with constant In content (20% or 30%) and experimental results were already mentioned in the above-mentioned publication. To obtain acceptable depletion zone lengths, layer thicknesses between 80 nm and 150 nm were prepared. However, these thicknesses are far greater than the corresponding critical layer thicknesses, so that the regions are already partially relaxed. Such structures contain a large number of dislocation lines and crystal defects, which exert a highly unfavorable effect on the electronic behavior of the diode as well as on the reliability and the quality of the component.
Even though it is possible, in principle, to prepare non-relaxed InGaAs layers on GaAs with high In content by drastically reducing the InGaAs layer thickness to values markedly below the critical layer thickness (e.g., below a thickness of less than 5 nm for an In concentration of 30%), this leads to an appreciable reduction of the electron barrier, because the conduction band jump between InGaAs and GaAs is very close to the Schottky contact and the edge of the conduction band of the GaAs material represents the effective barrier height for the current transport. Such a component consequently behaves like a diode with a GaAs depletion zone layer.
It has been known from K. Kajiyama, Y. Mizushima, and S. Sakata: Schottky Barrier Height of n-InxGal-xAs Diodes, Applied Physics Letters, Vol. 23, No. 8, pp. 458-459, 1973, that the electron release barrier in InGaAs material continuously decreases with increasing In content, beginning from pure GaAs material, and there is no barrier any more for pure InAs material. It has been well known that the lattice constant increases nearly linearly with increasing In content, which leads to a corresponding mismatch between InGaAs and GaAs. Electronic band jumps are formed at the interface of lattice-adapted and deformed (compression) InGaAs and GaAs, such that the conduction band in InGaAs is lower and the valence band is higher than in GaAs. It has also been known that the energy gap of elastically deformed InGaAs on GaAs decreases nonlinearly with increasing In content.
Other critical parameters of such diodes, especially in mixer arrangements, are the so-called cutoff frequency (fco), the ideality factor (n), and the noise factor of the component. These variables depend mainly on the nonlinear conductivity and the limiting, loss-containing equivalent circuit variables, namely, the series resistance (Rs), the junction capacitance (CjO), and the parasitic stray capacitance (Cpar), which should be minimized on the whole, and the relationship fco=[2.pi.Rs*(CjO+Cpar)].sup.-1 is valid.
An arrangement and a process for preparing planar millimeter-wave diodes has been known from D. G. Garfield, R. J. Mattauch, and S. Weinreb: RF Performance of a Novel Planar Millimeter-Wave Diode Incorporating an Etched Surface Channel, IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 1, pp. 1-5, 1991. The structure is based on a GaAs material layer sequence with relatively thick epitaxial layers. To reduce the parasitic capacitances, the authors use an etched channel structure and an air bridge lead for the Schottky metalization. Good CjO and Rs values can thus be reached, but the process steps are technologically complicated, and the structure also has the disadvantage that the parasitic coupling capacitance as a dominant component in Cpar between the Schottky metalization line and the conductive semiconductor layers separated via a dielectric layer is high. The arrangement is also unsuitable for the preparation of planar components.
An arrangement and a process for preparing planar Schottky mixer diodes has been known from B. Adelseck, A. Colquhoun, J. -M. Dieudonne, G. Ebert, D. -E. Schmegner, W. Schwab, and J. Selders: A Monolithic 60 GHz Diode Mixer and IF Amplifier in Compatible Technology, IEEE Transactions and MicroWave Theory and Techniques, Vol. 37, pp. 2142-2147, 1989. The component is prepared according to a MESFET technology by using the following process steps: Silicon implantation, high-temperature healing, epitaxial overgrowth, insulation implantation, and electron-beam lithography. The Schottky metal contact is sunk via a "recess" channel into the depletion zone layer, which is located buffed under a highly doped contact layer. Published limit frequencies of fco&lt;1.5 THz and cutoff voltages between 0.7 V and 0.8 V were reached (J. M. Dieudonne, B. Adelseck, E. E. Schmegner, R. Rittmeyer, and A. Colquhoun: Technology Related Design of Monolithic Millimeter-Wave Schottky Diode Mixers, IEEE Transactions and Microwave Theory and Techniques, Vol. 40, pp. 1466-1474, 1992). This process does not make it possible to prepare diodes with reduced cutoff voltages and to reach very low Rs values with simultaneously low CjO values, because the conductivity of the ohmic lead layer prepared by silicon implantation is limited, the current must flow through the low-doped depletion zone layer (PET arrangement), and the Schottky metal contact resistance already increases markedly in these small structure geometries (&lt;0.3 .mu.m) and it dominantly determines Rs.